This application claims the priority of German application 198 25 308.7, filed in Germany on Jun. 5, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a variable valve operating mechanism for an internal-combustion engine, in which a lifting valve is operated by a radial cam of a camshaft via a valve actuating lever. By way of a cam plate arranged in its end area away from the valve and by means of a roller which can be displaced in a controlled manner, the valve actuating lever interacts with the radial cam such that the roller, which can be discontinuously displaced transversely to the camshaft axis of rotation against, and in, the rotating direction of the radial cam, is used for changing the opening and closing points in time of the lifting valve. This is done in the case of constant opening angles in combination with variable valve lifts because of the change of the roller-caused valve actuating lever transmission ratios.
A variable valve operating mechanism of this type having a rocker arm is known, for example, from European Patent Document EP 0 717 174 A1. In this known valve operating mechanism, variable valve lifts are achieved by changing the rocker arm transmission ratio which occurs with the controlled change-of the position of the roller from a position close to the rocker axis to a position away from the rocker axis along the cam plate integrated on the rocker arm. As a result of the system, a relative change of the position of the roller with respect to the radial cam interacting with it is also connected with this rocker arm transmission ratio, which is achieved by way of the roller. This is done in such a manner that, in the case of a rocker arm transmission ratio in the direction of smaller rocking angles, the roller engages earlier with the opening flank, and thus the opening and closing points in time of the lifting valve are therefore adjusted to "early". A constant opening angle is connected with the above which exists between the respective opening and closing points in time of the lifting valve as the result of the dimensioning of the radial cam, which opening angle remains constant at each of the achievable reduced valve lifts.
It is a disadvantage of this known valve operating mechanism that, for achieving desired fluid dynamics in the combustion space of the internal-combustion engine for kinematic reasons, no sufficiently low valve lifts can be achieved on the one hand and, on the other hand, the valve opening angles remain constant.
It is an object of the invention to improve a valve operating mechanism of the above-mentioned type such that, particularly in the low load range of the internal-combustion engine, particularly small valve lifts can additionally be achieved at variably adapted valve opening angles.
According to the present application, this and other objects are achieved in that the rocker axis of the valve lever, which is essentially parallel to the axis of rotation of the camshaft, can be dynamically displaced relative to the camshaft along a path in a controlled manner. When the rocker axis is displaced along the path extending transversely to a straight line connecting the roller center with the valve lever support on the shaft end of the lifting valve, the valve lever is activated for the valve operation by way of the roller which is controlled only approximately from the starting area to the end area of the cam tip of the radial cam.
By means of the valve operating mechanism according to the invention, it can be achieved that, with an opening and closing flank of the radial cam moved under the roller fixedly positioned in a controlled manner, at first no valve operation takes place by way of the valve lever escaping because of the displaceable rocker axis. This valve operation will occur only when the roller only approximately from the starting area to the end area of the cam tip of the radial cam activates the valve lever for the valve operation in a controlled manner.
It is an advantage of the invention that the additionally respectively achievable variable valve opening angle is &lt;120.degree. with respect to the crankshaft at variable valve lifts &lt;1.5 mm, particularly when a rocker lever is used as the valve lever.
An advantageous embodiment of the invention has a rocker lever tiltably connected about its rocker axis connected with a lifting cylinder liftably guided along a straight path in a bearing bridge fixedly arranged on the housing. In this case, the lifting cylinder, which is acted upon by a prestressed spring at the end away from the rocker lever, includes in a guiding section, an oblong hole. The oblong hole is dimensioned corresponding to the predetermined lifting displacement of the rocker axis, for the engagement of a stop pin arranged in the bearing bridge. Additionally, a recess is provided in this guiding section for the engagement of an indexing pin which can be triggered for the release of the lifting cylinder which is driven to be liftable against the spring between the ends of the oblong hole in the area of the cam tip of the radial cam by way of the roller and the rocker lever which can be swivelled about the valve-stem-side rocker lever support.
In contrast to known variable valve operating mechanisms with several shiftable valve levers, the present invention has the advantage of a valve operating mechanism with only one rocker lever. In connection with the lifting cylinder, the rocking lever has fewer masses which must be moved than the known-prior art.
The above-described basic construction of the valve operating mechanism according to the invention advantageously permits a further development having a single camshaft with radial cams for the inlet and outlet lifting valves, comprising a rocker lever for an outlet lifting valve, which rocker lever is disposed on a rocker lever shaft arranged fixed to the housing. In this case, the control shaft is provided between the rocker lever shaft and the camshaft arranged, relative to the latter, deeper in a cylinder head. This control shaft operates the roller between the radial inlet cam and the rocker lever with the rocker axis, which can be displaced in a controlled manner, for the variable operation and control of the inlet lifting valve preferably in the lower load range of the internal-combustion engine, particularly in the partial load range close to idling.
In addition to an advantageous compact construction of the valve operating mechanism according to the invention, by means of this valve operating mechanism, the air flowing into a combustion space can, in a further advantageous manner, be provided with a swirling motion to form a better mixture.
In a further development of the invention, this applies particularly to a cylinder head with two inlet lifting valves per combustion space, the assigned rocker levers have a displaceable rocker axis, each of which is arranged to be separately controllable such that one of the lifting valves is variably controlled with a constant opening angle with respect to an inlet/outlet overlap and a valve lift, and the other lifting valve is variably controlled with a variable valve opening angle &lt;100.degree. with respect to the crankshaft with valve lifts &lt;1.1 mm.
As a result, by generating a swirl in the combustion space in a targeted manner, an improved combustion can be advantageously achieved, while reducing the fuel consumption.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.